Iron meteorite groups

Structure tells you what an iron meteorite looks like when etched, but chemistry tells you which parent body it came from. Here you will learn both systems.

Structural classes in detail

The structural classification uses the width of kamacite bands in the Widmanstätten pattern, which depends mainly on nickel content and cooling rate.

The classes and their usual abbreviations are: hexahedrite, H, with no Widmanstätten pattern; coarsest octahedrite, Ogg, with bands wider than about 3.3 millimetres; coarse octahedrite, Og, about 1.3 to 3.3 millimetres; medium octahedrite, Om, about 0.5 to 1.3 millimetres; fine octahedrite, Of, about 0.2 to 0.5 millimetres; finest octahedrite, Off, below about 0.2 millimetres; plessitic octahedrite, Opl, with tiny spindles of kamacite; and ataxite, D, with no visible structure.

This system is useful and easy to observe, but meteorites with very different origins can share a structure. That is why chemical groups became the main basis for classifying irons.

Chemical groups

From the 1950s onwards, researchers, most notably John Wasson and colleagues, measured trace elements in hundreds of iron meteorites. They found that when the elements gallium, germanium and iridium were plotted against nickel, irons fell into distinct clusters. Each cluster is a chemical group, thought to represent a separate parent body or core.

The groups have names like IAB, IIAB, IIIAB, IVA and IVB. The Roman numeral originally reflected gallium and germanium levels, with group I highest and group IV lowest. The letters subdivide them, and some groups originally thought separate, such as IIIA and IIIB, were later combined.

About a dozen or so groups are recognised. Perhaps 10 to 15 per cent of irons do not fit any group and are described as ungrouped, implying that many more iron parent bodies existed.

Magmatic and non-magmatic irons

Most groups, including IIAB, IIIAB, IVA and IVB, are called magmatic irons. Within each, trace element concentrations change systematically, which matches what is expected when a molten metal core crystallises gradually from the outside in. Elements such as iridium prefer the first solid metal to crystallise, so early-formed irons are rich in iridium and later ones poor in it. Within IIIAB, iridium concentrations vary by factors of hundreds or more.

Other groups, notably IAB and IIE, do not show these simple trends and often contain silicate inclusions. They are called non-magmatic irons and probably formed in pools of impact-melted metal or in partially melted bodies, rather than in a fully separated core.

The distinction matters because magmatic groups let scientists reconstruct how an entire asteroid core solidified, while non-magmatic irons record messier processes involving impacts and partial melting.

Famous examples

Knowing the groups helps you place famous meteorites in context.

IIIAB is the largest group of iron meteorites. Cape York, from Greenland, and Willamette, from Oregon in the USA, are IIIAB medium octahedrites.

IAB includes Canyon Diablo, the meteorite that formed Meteor Crater in Arizona, and Campo del Cielo from Argentina. Many IAB irons contain silicate inclusions related to the stony winonaites.

IVA includes Gibeon from Namibia and Muonionalusta from northern Sweden, both fine octahedrites.

IIAB includes Sikhote-Alin, which fell in eastern Russia in 1947 as a spectacular shower of iron fragments.

IVB includes Hoba, the largest known meteorite, a nickel-rich ataxite. IVB irons are depleted in volatile elements, suggesting their parent body formed in hot conditions or lost volatiles.

Did you know?